In Japanese Patent No. 4084248, brake control for a vehicle is described which sets a target slip ratio of a front outside wheel to a value higher than a normal value and controls the brake force at the front outside wheel based on the target slip ratio when a vehicle is under an avoidance maneuver operation in emergency, wherein the target slip ratio is higher than that in a spin control which works when a friction coefficient (hereinafter referred to as μ) of a surface of a road is high. More specifically, a correction coefficient is set to a higher value in order to correct the target slip ratio to a higher value as an inertia moment corresponding to an acceleration in the lateral direction of the vehicle (hereinafter referred to as a lateral acceleration) becomes larger.
FIG. 8 is a graph showing a relation between the slip ratio and the lateral acceleration of the front outside wheel. The characteristics of the graph depend on characteristics of a tire, state of a surface of a road, the total weight of the vehicle, and the like. FIG. 9 is a graph showing a relation between the lateral acceleration and a roll angle of the vehicle.
As shown in FIG. 8, the lateral acceleration reaches its peak value when the slip ratio becomes a certain value (approximately 5% in FIG. 8) and decreases as the slip ratio becomes larger beyond the certain value. As shown in a line 31 in FIG. 9, the lateral acceleration is proportional to the roll angle until the vehicle comes into a state where there is a possibility of the lateral overturn. Therefore, the lateral acceleration can be a parameter indicating the roll angle, which means that the roll angle becomes large as the lateral acceleration reaches its peak value. A region 32 is a region in which the lateral overturn is highly possible.
If the target slip ratio for the front outside wheel is set to a high value as in Japanese Patent No. 4084248, the target slip ratio becomes larger than a value of the target slip ratio at which the lateral acceleration reaches at its peak value. Therefore, the lateral acceleration can be suppressed if the actual slip ratio is controlled so that it becomes equal to the target slip ratio. FIG. 9 also shows an example of an anti-lateral overturn control in which the front outside wheel is forced to slip and the lateral acceleration accordingly decreases from a region 33 to a region 34.
However, the target slip ratio becomes larger as the inertia moment (i.e. lateral acceleration) becomes larger in Japanese Patent No. 4084248. Therefore, when the high target slip ratio causes the lateral acceleration to decrease, the correction coefficient changes in accordance with the decrease of the lateral acceleration, and the target slip ratio also changes to the lower value in accordance with the change of the correction coefficient. As a result, the lateral acceleration gets closer to its peak value in accordance with the decrease of the target slip ratio, and the roll angle accordingly becomes larger. When the roll angle gets larger, it becomes more likely that the roll angle reaches a roll limit value 30 at which the lateral overturn of the vehicle possibly occurs. In other words, the lateral overturn cannot be suppressed sufficiently. Especially in the case that the loading weight of the vehicle becomes larger and that the total weight of the vehicle accordingly becomes larger, the peak value of the lateral overturn in FIG. 8 also becomes larger. Therefore, it becomes more difficult to suppress the lateral overturn sufficiently.